1. Field of the Invention
The present invention relates to a control device for an internal combustion engine, and more particularly, to a control device for an internal combustion engine equipped with a mechanical supercharger including a compressor to be driven via a belt by an output shaft of the internal combustion engine.
2. Description of the Related Art
Hitherto, a control system for an internal combustion engine (hereinafter referred to as engine) equipped with a supercharger has been developed in order to increase an output of the engine. As an example of the supercharger, there have been known a turbo supercharger (hereinafter referred to as turbocharger, and also sometimes referred to as T/C) and a mechanical supercharger (hereinafter referred to as supercharger, and also sometimes referred to as S/C).
In a turbocharger, a turbine provided to an exhaust system of the engine is rotated at high speed by using energy of exhaust gas. With this, a compressor, which is provided to an intake system and is connected to the turbine, is driven.
Besides, in a supercharger, a compressor provided to the intake system of the engine is driven via a belt by using an output shaft of the engine.
In recent years, there have been developed an engine system having a plurality of turbo chargers disposed in series or in parallel and an engine system having both the turbocharger and the supercharger provided thereto. Further, there has been developed an electric charger for directly driving the compressor with an electric motor.
In the turbocharger, there is a possibility that excessive increase of a supercharging pressure at high rotation and high load operation could lead to a damage of the engine. Therefore, in general, an exhaust bypass passage is provided upstream of the turbine. Moreover, a part of the exhaust gas flowing through an exhaust passage is branched to a bypass passage by a waste gate valve provided to the exhaust bypass passage. In this way, the supercharging pressure is controlled to an appropriate level by adjusting an inflow amount of the exhaust gas into the turbine.
As a control method for the supercharging pressure by the waste gate valve, for example, there is known a control device for an internal combustion engine as described in Japanese Patent No. 5420013. According to Japanese Patent No. 5420013, first, a target intake air flow rate and a target charging efficiency are calculated based on an engine output target value. Then, a target throttle upstream pressure is calculated based on the target charging efficiency and a rotational speed. Further, a target compressor driving force to be required for driving a supercharger is calculated based on the target intake air flow rate and the target throttle upstream pressure. On this occasion, the exhaust gas flow rate is calculated based on an air-to-fuel ratio and the intake air flow rate, and characteristics of the exhaust gas flow rate and a compressor driving force only depend on a waste gate valve control value. A target waste gate valve control value is calculated by using this relationship from the exhaust gas flow rate and the target compressor driving force.
The control device for an internal combustion engine disclosed in Japanese Patent No. 5420013 is high in compatibility with the so-called torque-based control, which is a recent mainstream. Therefore, the control device for an internal combustion engine disclosed in Japanese Patent No. 5420013 has such excellent features that an acceleration response characteristic can be operated, an operation can be carried out at an optimal point of the fuel efficiency, and variation elements can be learned. It should be noted that the torque-based control is a control method involving setting an output shaft torque of the engine, which is a required value for a driving force by a driver or a vehicle, as an engine output target value, and determining an air amount, a fuel amount, and an ignition timing, which are main engine control amounts.
On the other hand, also in the supercharger, there is a possibility that excessive increase of a supercharging pressure at high rotation and high load operation could lead to a damage of the engine. Therefore, a bypass passage for bypassing the supercharger is provided. Then, a bypass valve provided to the bypass passage is used so as to return the air in a supercharger downstream to a supercharger upstream. As a result, the supercharging pressure is controlled to an appropriate level.
As another method in the supercharger, there is known a method of suppressing the supercharging pressure to an appropriate level by disconnecting the supercharger from the output shaft of the engine by means of an electromagnetic clutch.
As a control method for the supercharging pressure by using the bypass valve, for example, in Japanese Patent Application Laid-open No. H 4-325717 A1, there is disclosed an intake air control device for a supercharged internal combustion engine for calculating a target duty ratio of the bypass valve and a target intake air amount based on an output signal of a load sensor which generates an output voltage corresponding to a depressed amount of an accelerator pedal and the like, and applying feedback control to the target duty ratio by using the target intake air amount corrected depending on an environment and an intake air amount detected by an airflow meter. Moreover, in Japanese Patent No. 3366399, there is disclosed a supercharging pressure control device for a supercharged engine for calculating a bypass air amount for attaining a target supercharging pressure depending on an engine operation state, and determining a control amount for the bypass valve based on the difference between a downstream side pressure and an upstream side pressure of a supercharger.
However, in the control device disclosed in Japanese Patent No. 5420013, the control method for the supercharging pressure is carried out by means of the waste gate valve, and hence cannot be applied as the control method for the supercharging pressure by means of the bypass valve.
In Japanese Patent Application Laid-open No. H 4-325717 and Japanese Patent No. 3366399, there are disclosed the methods for the supercharging pressure by means of the bypass valve. However, in the control device disclosed in Japanese Patent Application Laid-open No. H 4-325717, it can be considered that the control system is constructed while assuming a one-to-one relationship between the target duty ratio and the target intake air amount corrected depending on the environment. In the control device according to Japanese Patent Application Laid-open No. H 4-325717, the relationship with the throttle opening degree or the throttle downstream pressure is not taken into consideration. Therefore, for example, when the throttle opening degree or the throttle downstream pressure changes due to variations in environmental conditions and the throttle valve and the like, and the relationship between the target duty ratio and the target intake air amount corrected depending on the environment thus no longer holds true, there is a first problem of degradation of controllability. Further, the control device according to the Japanese Patent Application Laid-open No. H 4-325717 has a second problem in that a torque requirement from sources other than the driver, such as a torque down requirement from transmission control, traction control, or the like cannot be dealt with. It is conceivable that those problems are solved by the control device disclosed in Japanese Patent No. 3366399. This is because, regarding the first problem, the control amount for the bypass valve is determined while accounting for the throttle opening degree and the throttle downstream pressure, and regarding the second problem, the control amount for the bypass valve is determined by using the usual target supercharging pressure as well as a target supercharging pressure for the traction control.
However, in the control device disclosed in Japanese Patent No. 3366399, it is considered that the control system is constructed based on the relationship among a flow rate (ABV passage flow rate) of air passing through the air bypass valve (ABV), a differential pressure before and after the ABV, and an ABV opening degree. However, in recent years, the supercharger is constructed integrally with the bypass valve, and the bypass valve is thus close to the supercharger. Therefore, air in the neighborhood of the bypass valve is disturbed depending on the rotational speed (∝ engine rotational speed) of the supercharger and the passage flow rate. As a result, an effective opening area of the ABV changes, and as a result, the relationship among the ABV passage flow rate, the differential pressure between before and after the ABV, and the ABV opening degree greatly change depending on the operation state. Therefore, in order to calculate or measure the relationship among the ABV passage flow rate, the differential pressure between before and after the ABV, and the ABV opening degree, and so as to precisely simulate the relationship thereof, maps more in number than those illustrated in FIG. 5 of Japanese Patent No. 3366399 may be necessary. As a result, there is such a problem that man-hours for data measurement and adaptation increase. Moreover, in Japanese Patent No. 3366399, learning of an S/C discharge amount for correcting the variations is described. However, a variation element may exist also in the bypass valve in addition to the supercharger, and hence there is also such a problem that the correction is not sufficiently carried out only by the learning of the S/C discharge amount.